Excluded required packages, when compiling srpm's on RHEL 8 Beta

I was using Mock recompiled from Centos 7 to build packages. So far minimal issues but having some serious issues with some perl dependencies. (I am suspecting probably something I have misconfigured with modules)

As an example, I used the cyrus-imapd SRPM, disabled cassandra (Lots of missing dependencies, and just wanted to test), which resulted in the above.

I also get the same issues with the commented out dnf command above.c

Another example, is rebuilding perl-Nagios-Plugin-0.36-7.el7.src.rpm,
Error:
Problem 1: cannot install the best candidate for the job
- package perl-Exporter-5.72-1000.el8+1703+3db27f7b.noarch is excluded
Problem 2: cannot install the best candidate for the job
- package perl-constant-1.33-1000.el8+1703+3db27f7b.noarch is excluded

and quits when best=1 is set in /etc/dnf/dnf.conf which is true in the mock build and that is why it dies.
However I can't reproduce it on Fedora 29 which has dnf-4.0.9-2.fc29.noarch so the issue may have been fixed in upstream dnf (or a related package).

Thank you very much, been trying to solve this for a month now. (best=0 in the mock config got around this issue). And straight DNF (ignoring installroot) is giving me similar warnings (but installing the packages)